The projects center on experiments designed to obtain information about the mechanism and regulation of ribosome biosynthesis in eukaryotes. The lower eukaryote, Neurospora crassa, is being used as the model organism since it is highly amenable to the combined genetical and biochemical approaches we are using. We are concentrating on two projects: 1) New cold-sensitive and heat-sensitive mutants will be isolated and these will be screened for conditional defects in ribosome or macromolecular synthesis. Any such mutants will be studied further to pinpoint the molecular event(s) involved; 2) A strain has been obtained that has two nucleolar organizer regions. Experiments will be done to confirm that twice the wild-type number of rRNA genes are present in the strain and to investigate whether mechanisms exist to decrease ("demagnify") the number of rRNA genes to the wild-type level. In addition the effect of extra rRNA genes on precursor-rRNA synthesis and processing will be examined.